There is one more class I forgot about, fairly rare, but used by NAD in their stereo amps, a Class H (or is it G--I've just had my late afternoon glass of red wine so I'm too lazy to go look it up). The NAD amps have quite modest continuous rated power output, about 50 watts per channel, but when musical dynamic peaks come along, a second set of voltage rails on the power supply momentarily switch in to generate up to twice as much power, 100 watts in this case, equivalent to 3 dB of headroom, to avoid clipping the peak signal.
Outlaw's M2200s are also Class G. It's a kinda neat design, considering that most listening is done with only and handful of Watts.
The difference between G and H is that G has two rails of fixed voltage, it just steps up to the higher one when the lower can't deliver the needed power. With H the second rail is variable, and follows the signal providing only what's needed. H is even more efficient, but has slightly higher distortion.
